Vehicles incorporating an automatic transmission typically include a torque converter disposed between the automatic transmission and an engine of the vehicle. In a first mode, the torque converter transmits rotational energy from the engine to the transmission to allow the transmission to rotate wheels of the vehicle. In a second mode, the torque converter receives rotational energy from the engine but prevents such energy from rotating the transmission and, thus, the wheels of the vehicle. The torque converter essentially acts as a fluid coupling between the engine and the transmission that allows the engine to drive the wheels of the vehicle via the transmission in the first mode while allowing the engine to continue running without driving the wheels of the vehicle (i.e., when the vehicle is stopped, for example) in the second mode.
The input to the torque converter from the engine rotates generally at a higher speed than an output of the torque converter. For example, a conventional torque converter may include an impeller directly driven by the engine and a turbine coupled to an input of the transmission and rotatably driven by movement of fluid within the torque converter caused by rotation of the impeller. The impeller typically rotates at a higher speed than the turbine during operation. This difference in speed between impeller and turbine is referred to as “slippage,” which directly affects performance of the vehicle, as the slippage rate dictates how far an accelerator must be depressed prior to a vehicle being moved from rest, for example. The degree of slippage may be controlled by selectively applying a force to a converter clutch disposed within the torque converter, which, when applied, causes rotational speed of the impeller to more closely approximate that of the turbine. Generally speaking, a high degree of slippage indicates a high torque transfer and a high torque multiplication. Such high slippage also results in high energy losses due to the friction loss associated with directing fluid from the impeller towards the turbine when operating at high speeds.
Conventional control systems may be used in conjunction with a torque converter to apply a form of feedback control. For example, a feedback control system using an error signal that measures slip across the converter clutch may be used to control a pressure of fluid disposed within the torque converter and, thus, the degree to which the converter clutch is applied. While conventional control systems adequately control slip between the impeller and the turbine, conventional control systems mainly employ feedback control and therefore are typically slow to react to a change in driving conditions.
For example, when an accelerator is depressed, the error measured across the converter clutch (i.e., the difference in speed between the impeller and turbine) is great relative to the desired slip speed. As such, some time is required to allow oil pressure to sufficiently build up within the torque converter and exert a force on the converter clutch to allow the turbine speed to approximate that of the impeller to drive the transmission and, thus, the turbine, at a desired slip speed. This increased time results in a delay in acceleration of the vehicle and/or an oscillation in slip speed, and therefore reduces the performance and efficiency of the torque converter and vehicle.